The present invention is directed, in general, to geophysical exploration and, more specifically, to a system and method for determining vertical fractures in a stratum using scattered vertical and horizontal shear modes.
Most geophysical techniques currently dealing with multi-dimensional seismic data do not discriminate between seismic energies of different orientations, such as the compressional energy or vertical and horizontal shear energies of reflected seismic data systems. In a typical multi-dimensional seismic survey, a multi-mode seismic energy generator may be used to generate a preponderance of one orientation of seismic energy relative to a particular orientation. Then a preponderance of energies orthogonal to the first but relative to the same orientation may also be generated. However, the orientation of the received seismic energy changes at each receiver station due to a difference in orientation between the seismic energy source and each receiver in a multi-dimensional seismic array.
Differently oriented seismic energies may also propagate differently through the subsurface strata based upon the characteristics of the subsurface strata. Anisotropies in the subsurface strata also impact the seismic energies of different orientations, especially shear wave energy. Anisotropic subsurface parameters may be found in the form of thin-bed strata, laminae and bed matrix grains or pores that have a preferential direction caused by deposition or tectonic stress. Another common form of anisotropic subsurface properties are subsurface fractures. Anisotropies cause subsurface parameters such as permeability, shear strength and seismic velocities to have different values in different directions.
Compressional energy waves may generate vertical shear energy waves at subsurface interfaces. Additionally, vertical and horizontal shear waves may acquire significant second-order properties in areas containing subsurface anisotropies that complicate the problem of intermingling but also offer opportunity for analysis if the energies could be segregated. However, the processing of such data is complicated due to the intermingling and therefore not easily discriminated into the differently oriented energies for each source-receiver azimuth. Also, the processing of these components is further complicated since the orientation of the operational modes of the seismic energy source do not generally correspond to the orientation of each receiver in the geophysical data acquisition array.
The mapping of subsurface features may be greatly enhanced by processing the differently oriented seismic energies in a way that accommodates their different attributes. This is especially true in an orientation specific to the azimuths defined by each seismic energy source and receiver pair. Additionally, important rock property information could be ascertained by comparing differences and similarities of the attributes of the appropriately oriented seismic energies.
Accordingly, what is needed in the art is a way to more effectively segregate and differentiate subsurface anisotropic situations, such as vertical fractures, in seismic surveying situations.
To address the above-discussed deficiencies of the prior art, the present invention provides a system for, and method of determining vertical fractures in a stratum using scattered horizontal shear and vertical shear modes. In one embodiment, the method includes segregating first order horizontal and vertical shear modes of a seismic energy wave into second order horizontal and vertical shear modes, determining a seismic energy wave corridor along a radial path between the seismic energy source and the seismic receiver, and gathering seismic data received by the seismic receivers within the corridor wherein the data includes horizontal and vertical shear components.
In another embodiment, segregating includes transforming seismic energy wave generated by a seismic energy source and received by a seismic receiver into radial/transverse coordinate space. In such embodiments, transforming may include orienting a first seismic receiver with a seismic energy source along a radial path between the first seismic receiver and the seismic energy source to obtain a vertical shear component, and aligning a second seismic receiver substantially transverse to the radial path to obtain a horizontal shear component.
In this particular embodiment, the first and second seismic energy receivers may be substantially orthogonal with respect to each other and transforming further includes orienting first and second seismic energy receivers such that the first seismic energy receiver is aligned substantially perpendicular to a reflected seismic energy wave having an angle of emergence to thereby maximize the vertical shear energy received by the first seismic energy receiver and the second seismic energy receiver is aligned substantially horizontal with the angle of emergence.
In another aspect of this particular embodiment, the seismic energy receiver further includes a third seismic energy receiver substantially orthogonal to the first and second seismic energy receivers and orienting first and second seismic energy receivers includes aligning the third seismic energy receiver with the reflected seismic energy wave.
In yet another embodiment transforming seismic energy includes transforming waves generated by a seismic energy source and received by a plurality of seismic receivers within the corridor into radial/transverse coordinate space. In such embodiments, gathering may include summing data received by the plurality of seismic receivers and dividing the plurality of seismic receivers into stacking bins. In another embodiment, determining includes determining an azimuth of the corridor with respect to the seismic energy source and a width of the corridor.
The present invention also provides, in another embodiment a system for determining anisotropy in a stratum using scattered horizontal shear and vertical shear modes. In an advantageous embodiment, the system includes a seismic energy source, a reflected seismic energy wave reflected from a subsurface interface and having horizontal shear energy and vertical shear energy associated therewith, first and second seismic energy receivers, wherein the first seismic energy receiver is aligned radially with the seismic energy source and wherein the second seismic energy receiver is aligned substantially transverse with the seismic energy source, a seismic energy wave corridor extending along a radial path between the seismic energy source and the first seismic energy receiver, seismic data received by the first and second seismic receivers within the corridor wherein the data includes first order vertical and horizontal shear modes that have been segregated into second order vertical and horizontal shear modes.
In another aspect, the present invention provides a method of exploring a subterranean feature with seismic energy. In one exemplary embodiment, the method includes generating a seismic energy wave toward a subterranean feature, reflecting the seismic energy from the subterranean feature to produce a reflected seismic energy wave having vertical and horizontal shear energy associated therewith, segregating first order horizontal and vertical shear modes of a seismic energy wave into second order horizontal and vertical shear modes, determining a seismic energy wave corridor along a radial path between the seismic energy source and the seismic receiver, gathering seismic data received by the seismic receivers within the corridor.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.